From multiple stars the transition is easy to star-clusters. These seem to embody completely the idea contained in germ in the former class of objects. They are collections, often on the grandest scale, of sunlike bodies small and large, united in origin and history, acted upon by identical forces, tending towards closely related ends. The manner and measure of their aggregation, however, vary widely, and with them the cogency of the evidence as to their organic oneness. There are innumerable cases in which it absolutely excludes doubt; there are some in which it is rather persuasive than convincing. It is not then always easy to distinguish between a casual ‘sprinkle’ of stars and a genuine cluster. Nor can the movement-test, by which so many physical have been discriminated from optical double-stars, be here applied. Internal displacements of a circulatory character have not yet become apparent in any cluster, and there is only one with an ascertained common proper motion.

This is the immemorial group of the Pleiades, famous in legend, and instructive, above all others, to exact inquirers—the meeting-place in the skies of mythology and science. The vivid and picturesque aspect of these stars riveted, from the earliest ages, the attention of mankind; a peculiar sacredness attached to them, and their concern with human destinies was believed to be especially close and direct.